He is fulfilling a prophecy. Maybe he has a different coloured skin from everyone else or he has unusual eyes. Maybe he wears a particular set of clothes or carries a particular weapon. Maybe he has some technology they don't know about that looks like something prophesied.

The prophesies predict his appearance and state that he will come and break the world and yet, at the same time unite it.

They don't want either.

He's the Harbinger of Doom

For some particular reason, this character is bringing the end times with him (chasly from UK's answer fits in here).

There's infinite ground to cover here. As a bonus, you can also create a character that ranges from a lawful good guy who, unknowingly, is bringing the end times (or even trigger them by accident) to a chaotic evil one that's actually trying hard to screw every living being in the planet (for motives of revenge, self preservation, power or just simply being an asshole).

He's Cursed

The great Wizardius is a very powerful mage known for casting the most despicable curses upon those who mess with him - and our hero had the bad luck of stepping on his favourite pet guinea pig.

As punishment, Wizardius banished him to another world/land. Shortly after his arrival, he discovered that (nearly) everyone in that place wants his head.

He is told by Unimportant Jenny that there's a spell (to which she's immune) that makes all living things that get near him get insanely filled with rage towards him.

He's in a Simulated Reality

Just picture the Matrix. Here, the character might or might not know he's in a simulation (if you ask me, things are much more interesting if he does but has to play by the simulation's rules to meet his goal).

Maybe our hero was trying to destroy this evil AI, but in order to do so, he had to get inside its own private world to beat it in its own game.

This is a good explanation as to why there's traps and various obstacles in his way that are not people/creatures (traps, mazes, etc).

People just hate his guts

Maybe our hero was once a very, VERY, bad ruler - heads, spikes, walls bad. Or maybe he's the heir of such ruler, a menace that terrorized the entire land for decades. When the opposing forces of the resistance finally got to overthrow this regime, they made it their personal goal to completely exterminate every member of the royal family.

Maybe they have golden shiny hair or some other unmistakable trait (again, chasly mentioned something like this) which is the reason why everyone knows who he is.

The point is that this guy is, did or was framed of something bad - and needs to run for his life, which would turn your game in a kind of hack'n'slash-survival thing.

In Harry Harrison's "Deathworld", this happens - all fauna on one planet is deadly aggressive toward outsiders. Toward the end of the book we find out why (based on my recollection):

the animals are psychic and can read the emotional state of the settlers, who don't belong there, and who feel afraid of the wildlife. The animals' proactive aggressive attitude is something of a learned defense mechanism and is not planetwide, but rather has worsened to an extreme near humans' settlements.

On your world it could be possible that the animals (be they smart or stupid) are psychic toward the emotional state of other animals, and intelligent off-planet outsiders stick out like sore thumbs to them and/or present as threats to the animals' existence. (Perhaps aliens have a "blank" or "hidden reading", and to the natives this is terrifying.) It would not be hard to suppose that as this trait evolved from a common ancestor, the default instinctual behavior toward aliens came with it and subtantially all animals on the planet will attack on mind-sight.

The explorer didn't go to any other world. He stays here on Earth all the time, he's just under the influence of Bath salts

Specifically, the aggressiveness of other people, wildlife and even inanimate objects is only from explorer's own perception. And people would really forget their own struggles in trying to catch and restrain the explorer.

Despite the fact this explorer is your protagonist, a common motivation for enemies to band together is if their shared land/world/place is threatened. This explorer has arrived and somehow begins to destroy/overpower/infect the world in which all these other characters inhabit. Perhaps he brings a virus/bacteria that their biology isn't immune to. Perhaps, like chasly from UK mentioned, he is fulfilling something that requires the destruction of their land.

Your character should pose some sort of threat. These inhabitants of the land would undoubtedly forget their usual tiffs and work together to save their home.

You can make the distinction that these inhabitants aren't exactly normal humans, wildlife, etc. in a normal land, and are instead are all living in a banshied realm akin to purgatory (which would probably make it easier to explain the demons and elementals anyway). All beings in this realm would have one thing in common, they were stripped of their souls when they were sent there.

When your explorer lands here bearing the only soul in the realm, it is certain to draw attention. The eminating lifeforce acts like an ultrasonic beacon drawing all of this lands inhabitants toward the source like zombies toward a sound. Whether these beings desire to have the soul, or find it's presense unbearable, they must destory the person who posseses it to release it.

Historically in our own world, the people living on a particular continent, island, or "undiscovered" piece of land have not always fared very well when explorers show up. Maybe this explorer is not the first one who has shown up in this land. Maybe they're the second or third or twentieth, and all the varied groups in this land remember how badly it went when strangers showed up the previous time(s).

For instance, maybe the last time a mysterious stranger showed up in this land they spread a disease that wiped out a huge percentage of local life, or enslaved multiple people from multiple groups, or re-drew a bunch of local boundaries on a map in a particularly clumsy way. Maybe all the power struggles and strife the peoples of this land are currently experiencing are a direct result of the last time some external explorer decided to get involved with their lives.

In this situation, I can believe that all the local groups would decide to prioritize "get rid of the outsider" over whatever their internal struggles are.

The explorer came from an area that everyone knows that is forbidden to get in and out. Whether be a cave or a desert, the inhabitants knew that ever since that area existed, no one had gotten out of it and thus forbid anyone from coming near it. An unknown creature exiting out this forbidden area, while the inhabitants are curious, they are also scared as it's possible that the explorer was the cause of the disappearance.

Another possibility: The inhabitants have evolved to have some empathetic ability, able to identify another inhabitant with their own "feelings". The explorer does not have this ability. Upon showing up, the inhabitants, when using their empathetic abilities, fail to feel anything from the explorer. Threatened by the unknown being that feels like an inanimate object, they attacked the explorer.

He is the chosen one

Prophecies have told about this day when an outside came to our lands bringing unwanted destruction (or maybe peace...) that nobody wants. If this hero completes its commit, the world as all we know will stop existing. And obviously, everyone has fear of changes, enough to surpass their differences an ally to destroy this greater evil.

Maybe there aren't prophecies, but knowing its currents actions, it is possible to figure out what is going to happen if he keeps working... on his quest.

Hate from the unknown

You said he is an explorer, which usually mean he is from further and (possibly) unknowns lands. Nobody knows much about him, and that is the greatest fear, nobody knows what he is capable of, so the only option is to destroy the unknown, before he destroys you.

Shared Hatred of Outsiders

A few generations ago, all the races of the natives lived in relative peace, despite their inherent differences. The different groups didn't love each other but they tolerated the presence of other groups.

Then a few outsiders came to this land and said they wanted to trade with the different groups. In this land, guests had always been treated with respect. All the groups agreed that the outsiders were a neutral party and allowed them trade across group boundaries. This land had many things that were highly valued back in the outsiders homeland. The trade deals made the outsiders very rich. They brought even more traders into the land and created trading outposts in the lands of all the groups. Then they brought in armed guards to protect their outpost and trading caravans. The natives frowned at the armed guards, but after reassurances from the traders they grudgingly allowed them.

But, the traders' greed was not satisfied with just trading. They saw the land rich with resources and they wanted it for themselves. They knew they could not have the land as long the natives were strong. So, they started to use their business relations with the different native groups to inflame the groups against each other. They created more dissent among the groups, all the while bringing in more "armed guards" and turning trading outposts into military holds.

By the time the native groups realized what was happening, they were already too weak to stop the outsiders from taking over. The outsiders removed (assassinated) strong native leaders and replaced them with puppet leaders dangling on outsiders' strings. And thus started the outsider rule.

Had they truly ruled the land, things would not have been too bad, but the outsiders never truly considered the land and its people as their own. They did not even see the natives as humans. The outsiders saw the land and its people as resources to be exploited and squeezed to fill their own pockets, regardless of what it did to the natives. The oppressed natives had become little more than slaves in their own land. This continued for a long time.

Slowly but steadily, the natives started resisting. Small rebel raids started harassing the outsiders. The raids turned into skirmishes and skirmishes into battles. The natives ignored their dislike of each other and united across race lines against their hated enemy - The Outsiders. Eventually, the Outsiders were overthrown and sent out of the land never to return again.

But the cracks created among the native groups ran too deep to be mended completely. Now that they did not have a common enemy anymore, they split into different nations and tensions started flaring again.

Until another Outsider stepped foot on their land.

The planet has an immune system, with all lifeforms being able to identify foreign bodies and try to eliminate them, just like the human body automatically attacking unidentified foreign cells.

Maybe that planet has been invaded often enough by foreign lifeforms that it developed an "immune system" of some sort. The Earth never having been invaded this way might explain why it lacks it.

Everything there is part of a single hivemind. Normally you don't see it much as it thinking about something might express itself as a tribe hunting a certain animal or a plant growing in a certain spot.

Kind of like a computer but rather than 0,1 we have plants animals peoples their state of being locations etc

When an outsider arrives they chop wood and are not part of the hivemind. How would you react to someone coming in and literally starting to alter your thoughts, memories etc. An all out assault with everything you have seems perfectly resonable

While it may seem weird, write the story narrative backwards. Start with where you want to end up, then decide what the climax is and how you get there, once you know that then the rest is often much simpler and an answer to your question is likely to present itself.

While a controlling force might be cliche, there is a reason that it gets used so often; it sells and it's simple. Lord of the Rings has Sauron and the One Ring, Diablo has the namesake and the other Prime Evils (Diablo III expands on this further making your character the offspring of Angels and Demons), Warcraft has the Burning Legion, Game of Thrones has power and control, Legend of the Seeker has magic.

That being said, the protagonist may not be a stranger at all, they may have been a former leader that went missing at a crucial moment, it was assumed that they deliberately left and they have now returned but the protagonist has no memory of that.

Sounds like Silent Hill 2 to me, where everything in the game is hostile to the protagonist, but it's also implied that the horrors he experiences are creations of his traumatized mental state, and it's all in his head.

So maybe the protagonist is a victim of their own violent delusion?

Read the plot outline of Silent Hill 2 if you're interested. I won't spoil anything here because the execution of the story is quite masterful, but if you don't feel like scaring the crap out of yourself you can just read the outline and take inspiration from that.

Something that's been used to similarly cause large numbers of people to react in the same fashion to a particular individual is pheromones. While that may not work as well in an alien environment, let's be less literal - something about how your character smells causes all life on this planet to react negatively to him/her.

This requires no advance knowledge of your character on the part of the local populace (unlike several of the answers I see here), and could reasonably be used to cause any lifeform with a sense of smell to react to him. (Up to you to decide if your undead have a sense of smell or not).

The explorer emanates psychic or electromagnetic fields which are out of phase with the new land. It happens only because of the characters origin being different from the rest of the characters. Alternatively, it could be a mutation specific to the character. Because of this, the locals are subconsciously influenced into a state of fight or flight, or even rage when the protagonist is near.

Explorer's Paranoia

What is Paranoia? Paranoia involves intense anxious or fearful
feelings and thoughts often related to persecution, threat, or
conspiracy [1].

This extends the trope of explorers traveling for extended periods of time in a claustrophobic environment (a plane, a boat, or a spaceship) prior to venturing on a new uncharted territory. Upon leaving the safety of their transportation device, it is possible that the difficulty of the exploration lead them to construct a delusional fantasy in which the outer world is the antagonist, and the only safe place is within the narrow walls of their cocoon.

Following the description of delusions and paranoia, there is no evidence that can convince these people of the contrary. Instead, it will be very easy for them to become even more strongly convinced deep in their beliefs. The temperature of a stream of water, instead of being refreshing, will be perceived as freezing; the sun, instead of being warm and welcoming, will become scorching and blinding; the fauna, instead of being curious or scared, will be threatening and aggressive; a spider-bite, instead of being just a bite, will lead to somatization of the fear of poisoning.

In a first-person, or a limited third-person narration, the reader has no obvious way to discern the truth from the delusional paranoia of the narrating voice. Hence, paranoia become reality, and delusion becomes truth.

As a biological cause, perhaps you can suggest they are all tied together in a sort of primordial hive mind that takes over when an outside influence invades. Be it telepathic, or pheromonal, or what ever. Perhaps they have this genetic memory where they were almost wiped out, and only those with this sort of religious fervor gene survived.

It'd be even more interesting to create a backstory on how the gene ended up in all the biological life on the planet.

What if the explorer(s) belong to a race, or have characteristics similar to a race, that had once existed in that foreign land. let´s call this race "The raiders". The raiders had an incredible power in that foreign land. Think of superman but evil. They were the ultimate overlords and legends speak of their horrible deeds, about how they brutalized and enslaved every race in the world. In ancient times all the other races united and defeated the raiders, but alas, they couldn´t destroy the raiders completely and were only capable of vanishing them to a foreign land, were their powers waned and they were, supposedly, incapable of scaping. If the explorers were part of the raiders, maybe they were only capable of reaching the foreign land when they gained space/portal technology; or their magical science had overcome their lack of enough magic, etc.

So the raider´s mere existence is hated and feared, for they were the true terrors of old. The legends you expect were never true.

Sorry about the grammar. -_-

EDIT

The shared characteristics should be easy to spot, maybe the existence of horns, a jewel in the forehead, skin color, tattoo patterns in the skin, or an omnious aura that every being in this realm is capable of feeling.

Maybe he indeed is doing lots of bad things, and he is so evil he manages to conceal this from himself.

The standard video game handwave is that

• they are sent, or otherwise incited by the Big Bad to kill the hero, or


• they actually aren't against the hero, or working together, for that matter, but the effect is still the same because
  
  
  
  
  • the hero is somehow in their way (e.g. by trespassing on their territory) or the other way round, or
  
  
  • they are just naturally aggressive (e.g. are predators/mutants)